Most vehicles have a seat belt assembly that limits passenger movement during a crash. Sometimes, a pretensioner is used to preemptively lock the seat belt before the crash occurs. Even when engaged, however, pretensioners do not prevent the seat belt from moving. For example, slack can accumulate when a seat belt is wound on a spool. Simply engaging the pretensioner does not eliminate the slack. Referred to as the film spooling effect, during a collision, the slack must be taken up before forward movement of the passenger can be stopped.